I have had my MacBook Pro for about 6 months; bought it new in 2010. I am running Mac OS 10.6.5.

Using Airport to connect to the Internet. Had no problems until this last week, when at an apparently random moment, some applications (e.g. Safari, iTunes, Adium and Tweetie) decide the Internet is no longer connected. The message from Safari is the standard "You are not connected to the Internet."

Pinging different hosts on the Internet works completely fine, and I can browse without any issue on Firefox.

Network Diagnostics reports that the Internet is connected. Turning Airport on and off, and closing and rebootting the applications doesn't seem to help.

I did manage to fix the problem with a reboot but would rather find the underlying issue and a better solution. Can anyone out there help?

Finally had the error occur again and tried your suggestion. Browsing by IP address works but browsing by name doesn't. Again, Firefox works fine for both, and 'host yahoo.com' from the terminal resolves the name correctly. Any ideas on how to fix, or further things to look at?
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Formulate Information DesignJan 18 '11 at 12:17

Cool. I think we've narrowed it down to DNS - for some reason your machine is not able to resolve the IPs to the websites. I do not have a reason as to why it does that but I would like to suggest that you change the DNS server entries on your network settings to 8.8.8.8 & 8.8.4.4 (these are Google DNS servers you can also use any free DNS services on the net eg. opendns) and test it out for a bit.
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deeviateJan 20 '11 at 2:30

Really appreciate all your attempts to help me with this one, deeviate. I know the DNS servers work because other computers on the network can resolve hosts. I also need local DNS servers to resolve internal addresses, so I can't use the Google ones.
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Formulate Information DesignMar 15 '11 at 6:21